1. Field of the Invention
This invention relates to fluorescent lamps and is directed more particularly to an amalgam assembly for use within a fluorescent lamp, and to a lamp having such amalgam assembly disposed therein.
2. Description of the Prior Art
The light output of fluorescent lamps is critically dependent upon mercury vapor pressure (vapor density) within the lamp envelope. The mercury vapor pressure, in turn, is controlled by the temperature of excess liquid mercury which condenses in the coldest part of the lamp envelope, the so-called xe2x80x9ccold spotxe2x80x9d. Fluorescent lamps typically include at least one tubulation that has an opening into the interior of the lamp envelope and which, in construction of the lamp, is used as an exhaust and fill tubulation. At completion of manufacture, the exhaust tubulation is hermetically tipped off and the tipped end typically becomes the lamp xe2x80x9ccold spotxe2x80x9d.
The amalgam is commonly located in the exhaust tubulation cold spot. Such amalgams reduce the mercury vapor pressure relative to that of pure mercury at any given temperature and thereby permit optimum light output at elevated temperatures. Such amalgams also provide a broadened peak in the light output versus temperature curve, so that near optimum light output is obtained over an extended range of ambient temperatures.
When lamps are operated at temperatures lower or higher than the optimum ambient temperature, light output decreases by as much as 30% or more relative to peak value. This is a common occurrence when lamps are operated in enclosed or semi-enclosed fixtures. In addition to reduced light output, the color of the light varies as a result of the varying contribution of blue spectral emission from the mercury vapor in the discharge.
The problem of mercury vapor pressure control under varying temperature conditions is solved, at least in part, through the use of various alloys capable of absorbing mercury from its gaseous phase. Alloys of low temperature melting metals are often placed within fluorescent lamps to amalgamate with the excess mercury, and to regulate the mercury vapor pressure within the lamp. Alloys known to be particularly useful in forming amalgams with mercury include a lead-bismuth-tin alloy, a bismuth-indium alloy, a bismuth and tin alloy, and a zinc, indium and tin alloy. Other useful amalgams may be formed with pure indium, pure lead, and pure zinc.
The lamp typically is provided with an excess amount of mercury amalgam, that is, more amalgam than is needed to supply the mercury vaporized when the lamp reaches a stabilized operating condition. As the lamp ages, some of the excess amalgam is required to replace the mercury chemically bound elsewhere in the lamp during the life of the lamp.
When an amalgam fluorescent lamp is turned off, the amalgam cools and the mercury vapor within the lamp is gradually absorbed into the amalgam. When the lamp is turned on, the lumen output is significantly reduced until the amalgam is warmed up to a point at which the amalgam emits sufficient mercury vapor to permit efficient lamp operation.
In some types of lamps, particularly electrodeless fluorescent lamps, it is important that the amalgam be prevented from settling within the arc environment in the lamp envelope where the amalgam can cause deleterious changes in the lumen output and the lumen-temperature performance of the lamp.
In base-up lamps, there has been a particular problem in that, in use, the sealed end of the tubulation is pointed upwardly and the end of the tubulation that opens into the lamp envelope is disposed downwardly of the amalgam. The amalgam has tended to drop by gravity downwardly into the lamp envelope, where a much higher temperature is present, causing a sudden rise in mercury vapor pressure and an increase in lamp voltage, resulting in the occurrence of black spots on the glass envelope. If the lamp voltage exceeds the maximum sustaining voltage of the ballast provided in the lamp, the lamp extinguishes. There is thus required a means for retaining liquid amalgam in the tubulation, but permitting mercury vapor to exit the tubulation and flow into the lamp envelope.
Accordingly, there is a need for an amalgam assembly including an amalgam retaining means for limiting the amalgam to the tubulation sealed end region, and for a fluorescent lamp provided with such an amalgam assembly.
An object of the invention is, therefore, to provide an amalgam assembly for disposition in an exhaust tubulation of a fluorescent lamp to prevent migration of liquid amalgam into the lamp envelope.
A further object of the invention is to provide an electrodeless fluorescent lamp having therein an improved amalgam assembly which prevents the aforesaid migration of liquid amalgam.
With the above and other objects in view, a feature of the present invention is the provision of an amalgam assembly for a fluorescent lamp. The assembly comprises a glass exhaust tubulation extending toward a base portion of the lamp, the tubulation being closed at an end adjacent the base portion, and a glass body disposed in the tubulation and retained by a pinched portion of the tubulation, the glass body being disposed between the pinched portion of the tubulation and the closed end of the tubulation. A mercury amalgam body is disposed between the glass body and the closed end of the tubulation. A mercury wetting metallic layer is disposed on at least one of (i) an inside surface of the tubulation between the pinched portion and the closed end of the tubulation, and (ii) a surface of the glass body, whereby to wet a selected one of (i) the interior surface of the glass tubulation and (ii) the surface of the glass body, respectively, to prevent the amalgam from flowing past the tubulation pinched portion and into the lamp envelope.
The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular devices embodying the invention are shown by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.